1. Field of the Invention
This invention relates to a document detecting apparatus for a copying machine, which judges whether or not an original document is placed on an original glass plate and determines said document size.
2. Description of the Prior Art
For such a conventional apparatus, there have been apparatuses disclosed in unexamined laid open 78849/1981, 48759/1982, 10172/1982 etc. for example. These apparatuses are adapted to determine an original document size on the original glass plate by detecting a reflected light from the original document surface.
In this reflected light detecting system, however, a measuring light is irradiated to the original document surface through the original glass plate and this reflected light is caught through the original glass plate again. Since the measuring light is reflected also by the original glass plate itself, there is a possibility that a detection error would arise due to ingress of the reflected light from the original glass plate.
Namely, since a surface of the original glass plate is approximately similar to a mirror surface, almost all parts of reflected light at the original glass plate surface reflect in a direction symmetric to a direction of incidence with respect to a normal line for the surface in a concentrated manner, as in case of reflection at a mirror surface (a light path of this case is called "major path" hereunder).
The original glass plate has two surfaces which reflect the measuring light in the above manner: One is a lower side surface where the measuring light enters the original glass plate and another is an upper side surface which contacts with the original document, and the major path exists for both surfaces respectively. These major paths are practically overlapped each other if said two surfaces are parallel.
In conventional apparatuses, a light receiving device adapted to catch the reflected light from the original document surface is installed in at least any one of said two major paths so that it may also catch the reflected light from the original glass plate. Therefore, the reflected light from the document surface could not be identified from the reflected light from the original glass plate depending on a original document surface condition, thus the original document would not be detected correctly.